I just first want to say that I think the grav barrel is an ingenious idea.

I came up with a way that could make this a gun a little more ergonomical (I just didn't like the whole "hold an inflator right up to the cartridge" schematic), but sustains the same modular capability for the co2 inflator. It might initially replace the pistol grip, but you could make a hollowed shroud to cover it.

You can pretty much tell from the picture what i mean. You would manually slide the tribarrel assembly back up to the valve coming from the tube which leads to the inflator. The inflater would be mounted inside some sort of pistol grip. The spring allows the barrels to pop out from the valve and self-rotate.